This invention relates to switch assemblies and more particularly to pushbutton keyboard switch assemblies for use in electronic pocket calculators, telephones or the like.
In coassigned U.S. Pat. No. 4,005,293, which issued on Jan. 25, 1977, a keyboard system is disclosed which employs a generally rectangular X-Y conductor matrix. As described in that patent a plurality of actuating elements are arranged in an array of columns and rows on one face of a substrate along with a plurality of conductors on the face of the substrate. The conductors are parallel to one another and extend in the direction of the rows. There is a conductor for each row of elements and another conductor for each column of elements. Each row conductor is disposed between the substrate and the elements of its respective row. Each of the elements is of a flexible electrically conductive material and has an initial position in which it is clear of its respective row conductor. Each of the elements is resiliently deformable from its initial position to an actuated position in which at least one point of the element is in contact with its respective row conductor. The elements in each column are electrically connected to their respective column conductor, whereby upon moving any one of the elements to its actuated position, a circuit is completed between the column conductor and the row conductor of the actuated element.
While keyboards made in accordance with the disclosure of the patent are very effective it has been found that in mass producing such keyboards yields could be improved if several modifications were effected. For instance, using mass production techniques difficulty occurred in placing and maintaining the conductors in their respective grooves within the required tolerance. That is, the location of the conductor wires relative to the top surface of the substrate frequently varied more than was desirable in order to obtain satisfactory yields in the manufacturing operation. Additionally it was found that the longevity of some of the devices was limited due to corrosion problems, that is, in certain environments a film could sometimes form on the conductors or elements thereby adversely affecting the electrical switching characteristics of the switches. In general it was felt desirable to find ways to improve the manufacturing yield as well as lower the cost without adversely affecting the reliability of the keyboard system and even improve its longevity.
Thus among the many objects of the invention may be noted the provision of a keyboard having conductor wires which are disposed in a desired location relative to the top surface of a substrate with a high degree of accuracy and consistency, to provide a switching assembly of the type described which is reliable, long lived and economical to produce. Yet another object is the provision of an improved switching device which is particularly adapted for easy and accurate assembly in a keyboard system. Other objects and features will be in part apparent and in part pointed out hereinafter.
Briefly, in accordance with the invention, a keyboard system comprises a switching assembly having a deformable electrically insulative substrate with a flat top surface and a plurality of grooves molded in the top surface and extending in a direction parallel to a given axis. Electrically conductive wires having a diameter d are disposed in the grooves which have a width no less than d and a depth greater than d. The wires are securely held in position by deforming portions of the substrate adjacent the grooves onto and around the wires. Flexible, electrically conductive strips having a column of dome shaped portions or discs overlie the wires. Each strip is electrically connected to a wire as by welding to the wire which has been formed with a platform area with a weld projection thereon. Each dome is formed with a centrally located downwardly extending projection which moves into engagement with a wire when the dome is depressed. In order to minimize corrosion problems materials are specially selected. In the preferred embodiment the strips are formed of high strength brass which has suitable electrical and physical properties for the flexible members, bus wires connected to the strips are formed of nickel containing bronze and contact wires with which the domes move into engagement are formed of nickel containing brass. The substrate is preferably formed with recesses beneath each dome to prevent interference of the domes with the substrate upon depression of the domes. A layer of electrically insulative, flexible material overlies the substrate and strips and is sealingly attached to the exposed top surface of the strips and substrate.
The wires extend beyond an end of the substrate and are connected to a transversely extending elongated bar which is disposed a selected distance from the free end of the wires. The bar is formed with a stop surface in order to facilitate placement of the wires to a desired depth in mating apertures of a motherboard. The bar is preferably made of deformable material so that the wires can be forced into the bar to be securely held thereby and is provided with a flash receiving area to prevent flash material from interfering with the stop surface.